Localization of sound is an important perceptual task that the human brain performs in many daily activities including talking with one another, driving automobiles, and working with machines. Relatively little is, however, known about the brain mechanisms of sound localization in man. To fill this gap has been the goal of neurophysiological and neuroanatomical studies of issues relevant to sound localization in laboratory animals. The integration of perceptual and neurophysiological studies is essential for the achievement of the goal. Owls offer an excellent model in which neurophysiological findings can be linked to sound localization. The studies conducted so far have identified distinct brain areas and pathways for processing of acoustic cues for sound localization. The present research addresses cellular, systems, and behavioral level problems to gain deeper insights into the brain mechanisms of sound localization. Specifically, attempts will be made to obtain answers or clues to the role of inhibition in the processing of ineraural time differences in nucleus laminaris. Theoretical analysis predicts that as sound intensity increases coincidence detectors without inhibitory input should lose their ability to discriminate between different interaural time disparities. This hypothesis will be tested by blocking inhibition while nucleus laminaris neurons are responding to variations in interaural time difference. Human beings can fuse two sounds delivered by earphones into a single phantom image. The distinctiveness of the image depends on the degree of similarity between the two sounds. In owls, this phenomenon can be studied behaviorally as well as neurophysiologically. Neurons of the owl's external nucleus of the inferior colliculus can detect interaural time differences even when noises delivered to the two ears are quite dissimilar. The present project determines the degree of similarity in noises that the owl needs to perceive a phantom source. Owls turn their heads in the direction of real and perceived phantom sources, but they may cease or refuse to do so under some circumstances. The question is whether this phenomenon is associated with silencing of the loci in the owl's auditory space map that code for the unattended sound sources. This question is answered by recording neural activities in the auditor space map of the behaving owl.